Ecology of the California spotted owl: breeding dispersal and associations with forest stand characteristics in northeastern California

Abstract

The California spotted owl (Strix occidentalis occidentalis) inhabits coniferous and hardwood forest in California and selects stands dominated by large trees with moderate-high canopy cover for roosting and nesting. It is also found in areas dominated by medium-sized trees or low canopy cover. The owl has been at the center of political and administrative debates due to its association with commercially valuable forest. This study examined the demographic response of spotted owls to cover type composition within owl territories. Vegetation was mapped and classified within 2.4 km radius circles around 67 spotted owl sites in northeastern California. Vegetation polygons were classified by dominant tree size class, canopy cover, and density of remnant trees >76 cm diameter at breast height (dbh). I aggregated resulting classifications into cover types and used cover type composition within the different owl sites to explain variation in site occupancy, apparent survival probability and reproductive output, from 1993-1998, and nest success from 1990-2000. All analyses included data representing 2 spatial scales: core area (814 ha) and nest area (203 ha). Site occupancy was positively associated with the amount of the nest area dominated by large trees and high canopy cover within the nest area, and negatively associated with the amount of non-habitat and the amount of area dominated by medium-sized trees with high canopy cover. Furthermore, site occupancy decreased over time and with increasing elevation. Apparent survival probability varied annually and was positively related to the area of each cover type multiplied by the quotient proportion used / proportion available for each type, at both the nest and core scales. Reproductive output was negatively related to elevation and non-habitat within the nest area. Nest success was positively associated with the presence of large remnant trees within the nest stand. These findings reveal that although the California spotted owl is found in a variety of forest stand types, site occupancy, apparent survival and nesting success were all enhanced by increased amounts of forest cover types known to be selected by the owl at the landscape scale. Reproductive output decreased with increasing amount of non-habitat. Spotted owls are territorial, generally non-migratory, and strongly philopatric. Nevertheless, California spotted owls exhibited breeding dispersal during 7% of between-year observations of banded individuals (n = 54 of 743 occasions). Median breeding dispersal distance was 7 km (range = 1-33). Breeding dispersal probability was higher for paired owls which lost their mates, single owls, owls at lower quality sites, younger owls, and owls which failed to reproduce in the year preceding dispersal. To a lesser extent, dispersal probability was higher for males than females. I found no strong relationships between dispersal distance and any of the conditions that were associated with dispersal probability. Dispersal resulted in improved territory quality in 72% of cases. Single owls and those which lost their mates were usually paired following dispersal.